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List:       linux-netdev
Subject:    [PATCH] [PKT_SCHED]: improve hashing performance of cls_fw
From:       Thomas Graf <tgraf () suug ! ch>
Date:       2005-04-07 13:09:25
Message-ID: 20050407130925.GU26731 () postel ! suug ! ch
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Some test results for the new hashing

Test-1: hash 0..2147483648

old 256 buckets hash (enum):
empty buckets: 0 average chain length: 8388607.996 min: 8388607 max: 8388608

new 1024 buckets hash (enum):
empty buckets: 0 average chain length: 2097151.999 min: 2097151 max: 2097152

Test-2: hash 65536*rand()

old 256 buckets hash (rand):
empty buckets: 0 average chain length: 256.000 min: 222 max: 296

new 1024 buckets hash (rand):
empty buckets: 0 average chain length: 64.000 min: 40 max: 93

Test-3: hash 1024*rand()

old 256 buckets hash (rand):
empty buckets: 3 average chain length: 4.047 min: 0 max: 9

new 1024 buckets hash (rand):
empty buckets: 380 average chain length: 1.590 min: 0 max: 5

Test-4: total time for 2147483648 hashes, 10 runs

      old hashing algorithm    new hashing algorithm
             1.619s                     1.610s
             1.623s                     1.611s
             1.625s                     1.612s
             1.629s                     1.614s
             1.633s                     1.616s
             1.637s                     1.621s
             1.638s                     1.625s
             1.639s                     1.630s
             1.644s                     1.632s
             1.737s                     1.658s


Please do a

	bk pull bk://kernel.bkbits.net/tgraf/net-2.6-cls_fw

This will update the following files:

 net/sched/cls_fw.c |   31 +++++++++++++++++++++++++++----
 1 files changed, 27 insertions(+), 4 deletions(-)

through these ChangeSets:

# This is a BitKeeper generated diff -Nru style patch.
#
# ChangeSet
#   2005/04/07 14:23:09+02:00 tgraf@suug.ch 
#   [PKT_SCHED]: improve hashing performance of cls_fw
#   
#   Calculate hashtable size to fit into a page instead of a hardcoded
#   256 buckets hash table. Results in a 1024 buckets hashtable on
#   most systems.
#   
#   Replace old naive extract-8-lsb-bits algorithm with a better
#   algorithm xor'ing 3 or 4 bit fields at the size of the hashtable
#   array index in order to improve distribution if the majority of
#   the lower bits are unused while keeping zero collision behaviour
#   for the most common use case.
#   
#   Thanks to Wang Jian <lark@linux.net.cn> for bringing this issue
#   to attention and to Eran Mann <emann@mrv.com> for the initial
#   idea for this new algorithm.
#   
#   Signed-off-by: Thomas Graf <tgraf@suug.ch>
#   Signed-off-by: David S. Miller <davem@davemloft.net>
# 
# net/sched/cls_fw.c
#   2005/04/07 14:22:58+02:00 tgraf@suug.ch +27 -4
#   [PKT_SCHED]: improve hashing performance of cls_fw
# 
diff -Nru a/net/sched/cls_fw.c b/net/sched/cls_fw.c
--- a/net/sched/cls_fw.c	2005-04-07 14:55:55 +02:00
+++ b/net/sched/cls_fw.c	2005-04-07 14:55:55 +02:00
@@ -46,9 +46,11 @@
 #include <net/act_api.h>
 #include <net/pkt_cls.h>
 
+#define HTSIZE (PAGE_SIZE/sizeof(struct fw_filter *))
+
 struct fw_head
 {
-	struct fw_filter *ht[256];
+	struct fw_filter *ht[HTSIZE];
 };
 
 struct fw_filter
@@ -69,7 +71,28 @@
 
 static __inline__ int fw_hash(u32 handle)
 {
-	return handle&0xFF;
+	if (HTSIZE == 4096)
+		return ((handle >> 24) & 0xFFF) ^
+		       ((handle >> 12) & 0xFFF) ^
+		       (handle & 0xFFF);
+	else if (HTSIZE == 2048)
+		return ((handle >> 22) & 0x7FF) ^
+		       ((handle >> 11) & 0x7FF) ^
+		       (handle & 0x7FF);
+	else if (HTSIZE == 1024)
+		return ((handle >> 20) & 0x3FF) ^
+		       ((handle >> 10) & 0x3FF) ^
+		       (handle & 0x3FF);
+	else if (HTSIZE == 512)
+		return (handle >> 27) ^
+		       ((handle >> 18) & 0x1FF) ^
+		       ((handle >> 9) & 0x1FF) ^
+		       (handle & 0x1FF);
+	else if (HTSIZE == 256) {
+		u8 *t = (u8 *) &handle;
+		return t[0] ^ t[1] ^ t[2] ^ t[3];
+	} else 
+		return handle & (HTSIZE - 1);
 }
 
 static int fw_classify(struct sk_buff *skb, struct tcf_proto *tp,
@@ -152,7 +175,7 @@
 	if (head == NULL)
 		return;
 
-	for (h=0; h<256; h++) {
+	for (h=0; h<HTSIZE; h++) {
 		while ((f=head->ht[h]) != NULL) {
 			head->ht[h] = f->next;
 			fw_delete_filter(tp, f);
@@ -291,7 +314,7 @@
 	if (arg->stop)
 		return;
 
-	for (h = 0; h < 256; h++) {
+	for (h = 0; h < HTSIZE; h++) {
 		struct fw_filter *f;
 
 		for (f = head->ht[h]; f; f = f->next) {

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